The invention relates to solid oxide fuel cell (SOFC) stacks and, more particularly, to an interconnect structure that enhances the lifetime of SOFC stacks.
A fuel cell is a device which electrochemically reacts a fuel with an oxidant to generate a direct current. The fuel cell typically includes a cathode, an electrolyte and an anode, with the electrolyte being a non-porous material positioned between the cathode and anode materials. In order to achieve desired voltage levels, such fuel cells are typically connected together using interconnects or bipolar plates to form a stack, or fuel cell stack, through which fuel and oxidant fluids are passed. Electrochemical conversion occurs, with the fuel being electrochemically reacted with the oxidant, to produce a DC electrical output.
The basic and most important requirements for the interconnect materials on the cathode side of a SOFC stack are sufficient oxidation and corrosion resistance in air at the stack operating temperatures; sufficient electron conductance; and close matching of thermal expansion behavior to that of the ceramic cell. In the case of metallic interconnects, the requirement of sufficient electron conductance is essentially equivalent to the electron conductance of the oxide scale that forms on the metal surface because the oxide scale tends to be the limiting resistance. Currently, the lack of stable, long-life (>40,000 hours), metallic interconnects for the cathode side of the stack, is a serious weakness of planar solid oxide fuel cells, because existing metal alloys cannot meet the thermal expansion, oxidation resistance, and electron conductance requirements simultaneously.
Cathode interconnect materials that have been used to date include perovskite-based ceramics, e.g. lanthanum chromite, high temperature chromium-based alloys or composites thereof, and nickel-based alloys or intermetallics have been used typically for cells operating in the 800–1000° C. range.
The prior art on ceramic-based interconnects such as lanthanum chromite indicates that this material exhibits both usable high temperature conductivity and thermal expansion behavior that matches the cell. However the ceramic is very expensive, has low toughness and is difficult to manufacture as a suitable interconnector. Chromium-based interconnector materials have similar drawbacks.
Lower operating temperatures, (650–800° C.) with planar anode-supported (ASE) cells, permit lower cost materials such as ferritic stainless steels that have a better coefficient of thermal expansion (CTE) match with the cell than Ni-based alloys. Commercial grades of ferritic steels may have suitable oxidation resistance at temperatures less than about 600° C. or for short lifetimes, but do not have the required oxidation resistance to last for 40,000 hours, or longer, due to the increasing ohmic resistance across the oxide scale with time under load.
The majority of prior art on these issues has attempted to prevent or ameliorate the degradation caused by oxide scale. Specifically, to take advantage of the lower cost and favorable CTE of ferritic steels, minor alloying additions and/or surface coatings have been researched to improve the oxidation resistance and conductivity. Certain elements such as Mn, appear beneficial in forming manganese chromite which increases the conductivity of the oxide scale, but more data are needed to determine whether both conductivity and oxidation resistance are sufficient for long-term applications. However, elements known to improve oxidation resistance, such as Al and Si, also tend to reduce the oxide conductivity and increase the CTE of the alloy. In Fe—Cr—Al—Y type steels, excellent oxidation performance is traded for the high resistivity of the alumina film. Hence, the current state-of-the-art with regard to low cost Fe—Cr-based steels, has not fully resolved the long-term contact and oxidation issues.
Other materials, such as Ni—Cr or Ni—Cr—Fe-based alloys, while having good oxidation/corrosion resistance by design, typically have CTE values in the 15–18 parts per million (ppm)/° C. compared to the 10–12 ppm/° C. of ferritic steels which better match the CTE of the ceramic cell.
Preferential removal of the oxide and/or coating/doping of the alloy surface with noble metals such as Ag, Au, Pt, Pd, and Rh has been used to mitigate conductivity loss by reducing oxygen diffusion into the contact points of the interconnect, but noble metals are too costly to use in power plants and commercial applications.
The oxidation resistance is clearly a concern on the cathode/oxidant side of the interconnect. However, the partial pressure of oxygen at the anode/fuel electrode may also be high enough to form Cr2O3 and the oxide may be even thicker (viz. the presence of electrochemically formed water) than on the cathode side of the interconnect, so the resistivity of the interconnect may increase on both sides. The construction materials on the anode side of the interconnect could be the same as the cathode, although prior art has shown that, in the case of a ferritic steel interconnect in contact with a nickel anodic contact, weld points that formed between the steel and the nickel still formed a thin electrically insulating Cr2O3 layer over time which degraded performance.
It is clear, from the above review of background art, that the need remains for a substantially improved interconnect between adjacent cells, whereby interface strains, caused by CTE mismatch during thermal cycling, are substantially eliminated, while the material provides long-term oxidation resistance and high electron conductance across the oxide scale. It is therefore the primary object of the present invention to provide an interconnect or bipolar plate that meets the aforementioned needs.
Other objects and advantages of the present invention will appear hereinbelow.